We describe a device compatible with CMOS technology, which permits to detect a single charge by multiplying it to an easily measurable level. The device is a bipolar transistor optimized for the operation in the Geiger avalanche mode. A single electron, injected through the base-emitter junction, triggers avalanche breakdown in the collector-base junction. The breakdown is then rapidly stopped by a quenching circuit. This cycle produces a voltage pulse at the collector, which corresponds to the injection of a single electron. By counting the pulses, currents down to the atto-Ampere range are measured. The device can be used as a bitstrearn-generator in a delta-sigma analog-to-digital converter.